This invention relates to honeycomb structures having improved resistance to damage from asymmetric thermal shock, the cells of which have anisotropic Young's moduli in planes perpendicular to their central longitudinal axes (hereinafter referred to as "cross-sectional planes"), and in particular to improved designs for heat recovery wheels using square, anisotropic cells.
Thin-walled cellular or honeycombed structures are desirable for many uses and in particular for uses involving the flow of hot gases therethrough, such as catalytic reactors and heat recovery wheels [also known as rotary heat exchangers]. Such structures consist primarily or entirely of a honeycomb matrix formed from a plurality of hollow, open-ended cells, the central longitudinal axes of which are generally aligned parallel to one another so as to permit the passage of gases in a uniform direction through the structure. These structures are operated under severe thermal shock conditions and are generally fabricated of ceramic or glass-ceramic materials having very low coefficients of thermal expansion so as to minimize thermal shock damage. Other materials, such as glass, cermet or other ceramic based materials could conceivably be employed if they have sufficiently compatible properties [e.g., strength, chemical resistance, refractoriness, abrasion resistance, etc.] for the particular service conditions involved.
Matrices of hollow, open-ended cells can be produced by the processes of "wrapping" (building up of corrugated layers) or extrustion. The larger size heat recovery wheels needed for efficient industrial heat recovery uses [typically two feet (61 cm) or more in diameter] have been formed previously by cementing together smaller matrices or cellular segments made by the wrap process. Because of the severe thermal conditions encountered in use [typically, instantaneous exposure to gases at temperatures as high as 1500.degree. Fahrenheit (about 820.degree. Centigrade), cyclic asymmetric heating at approximately 20 cycles per minute, 10,000 hours operation] these wheels were formed from material having very low coefficients of thermal expansion, generally on the order of 10.times.10.sup.-7 /.degree.centigrade or less over the range 0.degree. to 1,000.degree. centigrade, so as to resist damage. Wheels constructed by these prior methods with materials having greater coefficients of thermal expansion (for example, approximately 20.times.10.sup.-7 /.degree.centigrade or more over the range 0.degree.-1,000.degree. centigrade) have inevitably failed when operated under these conditions.
It is known that the thermal shock resistance of a honeycomb structure can be improved by such techniques as forming cells having movable expansion joints, as is disclosed, for example, in U.S. Pat. Nos. 4,135,018 and 4,127,691, and by providing discontinuities through the cell structure, as is disclosed in U.S. Pat. No. 3,983,283. It has been found that such cellular designs are relatively fragile and often difficult and expensive to fabricate successfully.
Matrices of cells formed by parallel, intersecting planes of material which extend across the matrix to form several adjoining cells are generally stronger and easier to successfully fabricate than the aforesaid flexible cellular designs. The use of uniformly oriented square cells or alternately oriented equilateral triangular cells of identical size to form a honeycomb structure is well known, especially in the area of extruded catalytic reactors. However, the inventors have found that heat recovery wheels made from bonded cellular segments frabricated in such fashion have failed when subjected to the aforesaid typical operating conditions.
As used herein, "temperature gradient" refers to the instantaneous temperature change occurring in a direction through a material, or, in other words, the differential of the temperature distribution curve through the material with respect to a given direction. The "direction" of the temperature gradient is the direction in which the gradient is measured, or, in other words, the direction with respect to which the differential is taken. Also as used in the application, "temperature difference" with respect to a point on a structure refers to the difference between the maximum and the minimum temperatures occurring at any given time along an axis passing through the point and across the surface of the structure. The direction associated with the temperature difference is the direction of the axis along which it was determined.
The terms "cross-section" and "cross-sectional plane" as used herein in referring to a cell refer to a view and plane, respectively, which are perpendicular to the central longitudinal axis of the cell.